Global efforts to extract energy from sewage in forms such as heat, biogas and even electricity may get a boost thanks to the work of a team of biochemists and microbiologists from Ghent University in Belgium, who are collaborating on a pilot project with DC Water in Washington DC.
Sewage from bathrooms and kitchens is a potential energy source because it contains various organic substances suspended in wastewater. If we want sewage treatment to be truly self-sustaining, the trick will be to find an efficient way to separate the organic matter from the wastewater – that way the wastewater can be recycled, and the organic matter can be used to generate bioenergy.
Currently, the overall principle of most sewage treatment plants revolves around optimizing the way microorganisms such as bacteria, fungi and protozoans feed on the organic contaminants in wastewater. As the microorganisms eat the organic matter, they form particles that clump together and settle at the bottom of a tank, allowing a relatively clear liquid to be separated from the solids and further purified.
This often includes a step called "contact stabilization," which involves using two aeration tanks to ensure the microorganisms are as active as possible before introducing them to the next batch of effluent needing treatment.
At the moment, the overall sewage treatment process recovers around 20 to 30 percent of the organic matter within the sewage mix. Dr Francis Meerburg, a researcher on the Belgian project, said their aim was to improve the way bacteria captures organic material.
"We periodically starve the bacteria, in a kind of 'fasting regimen'," explains Professor Nico Boon. "Afterwards, wastewater is briefly brought into contact with the starved bacteria which are gluttonous and gobble up the organic matter without ingesting all of it. This enables us to harvest the undigested materials for the production of energy and high-quality products. We [then] starve the rest of the bacteria, so they can purify fresh sewage again."
This new method can recover more than 55 percent of the organic matter from the sewage, which is a big improvement over current rates of 20 to 30 percent. According to the team's calculations, this amount should provide enough energy to completely treat sewage without the need for external electricity sources.
"We're not going to solve climate change with our process, but every bit helps," Vlaeminck says. "For comparison: in our region of six million people (in Flanders), the energy usage of our sewage treatment municipality, Aquafin, corresponds to the residential electricity use of more than 690,000 people (more than 10 percent of the population). This gives an idea on the energy saving potential and impact, if all sewage treatment would be energy neutral."
As a clear sign that there's a strong appetite for more efficient, affordable and sustainable processes in wastewater treatment, the team's work has gone directly from the lab to a large-scale application in the USA's capital city.
The researchers are currently collaborating with DC Water (the District of Columbia Water and Sewer Authority) to implement the new process on a part of the plant's full-scale water treatment installation. The next step is to evaluate how well the process can help achieve more efficient wastewater treatment on a large scale.

PubMed | AquafinType: Journal Article | Journal: Water science and technology : a journal of the International Association on Water Pollution Research | Year: 2011

An ASM2da model of the full-scale waste water plant of Bree (Belgium) has been made. It showed very good correlation with reference operational data. This basic model has been extended to include an accurate calculation of environmental footprint and operational costs (energy consumption, dosing of chemicals and sludge treatment). Two optimisation strategies were compared: lowest cost meeting the effluent consent versus lowest environmental footprint. Six optimisation scenarios have been studied, namely (i) implementation of an online control system based on ammonium and nitrate sensors, (ii) implementation of a control on MLSS concentration, (iii) evaluation of internal recirculation flow, (iv) oxygen set point, (v) installation of mixing in the aeration tank, and (vi) evaluation of nitrate setpoint for post denitrification. Both an environmental impact or Life Cycle Assessment (LCA) based approach for optimisation are able to significantly lower the cost and environmental footprint. However, the LCA approach has some advantages over cost minimisation of an existing full-scale plant. LCA tends to chose control settings that are more logic: it results in a safer operation of the plant with less risks regarding the consents. It results in a better effluent at a slightly increased cost.